1. Field of the Invention
This invention relates to planar dual-mode filters and to a method of construction of said filters and more particularly to dual-mode planar lumped element or distributive element filters having a thin film on a substrate. The film can be a metallic material such as gold, silver or copper or it can be a ceramic material that becomes superconductive at cryogenic temperatures.
2. Description of the Prior Art
The use of two degenerate modes in microstrip rings and patches to realize dual-mode resonators is known (see a book entitled "Planar Circuits for Microwaves and Light Waves" by T. Okoshi, published in 1985 by Springer-Verlag, pages 36 to 39). See also an article by Wolf entitled "Microstrip Bandpass Filters Using Degenerate Modes of a Microstrip Ring Resonator", Electron LETT, 1972, pages 163 and 164 and further a book entitled "Handbook of Microstrip Antennas" by James, et al., published by Peter Peregrinus Ltd. in 1989, pages 221, 222 and 273. Dual-mode filters made from ring resonators are described in Griffin, et al., U.S. Pat. No. 4,488,131 entitled "MIC Dual-Mode Ring Resonator Filter" and in an article by Guglielmi entitled "Microstrip Ring Resonator Dual-Mode Filters" distributed at a workshop on microwave filters for space applications by European Space Agency/ESTEC in June of 1991. This prior patent and articles describe dual-mode microstrip resonator filters having a structural discontinuity at a 45.degree. angle to the two orthogonal modes.
Fiedziuszko, et al., U.S. Pat. No. 5,136,268 describes a dual-mode planar filter having two or more resonators with a coupling path between resonators being straight or curved, a width of the coupling path being constant over its entire length. The resonators are square resonators with one corner cut-away at a 45.degree. angle to introduce a structural discontinuity. The Fiedziuszko, et al., U.S. Pat. No. 5,172,084 describes a planar dual-mode filter having circular resonators.
A major concern with known patch resonator filters is the difficulty in eliminating undesired coupling between patch resonators that are not interconnected by a coupling path. When this undesirable coupling occurs, the filters cannot be made to realize symmetrical frequency characteristics. Further, known patch resonator filters permit the realization of a relatively narrow bandwidth; or, they have a relatively high loss performance; or, they require the use of tuning elements to achieve the desired coupling.